The present invention relates generally to methods and apparatus for therapeutic treatment and/or monitoring of functions of the human or animal body, in part by means of the efficient transmission or delivery of electrical signals between a therapeutic or monitoring system, such as an automatic implantable cardioverter/defibrillator, and the tissue or blood of the body, such as that of the heart, via one or more implanted electrodes. More particularly, the invention is directed to improvements in the configuration, energy transfer efficiency, therapeutic and sensing effectiveness, and implantation techniques of such implantable electrodes, especially for purposes of defibrillation, and which in the preferred applications are composed of carbon fibers or carbon-coated metallic fibers, for stimulating or sensing electrical reactions in body tissue and especially heart tissue.
Many parallels exist between the cardiac therapy provided with implantable defibrillators and the cardiac therapy provided with implantable pacemakers. At the outset of the use of each of these devices, problems were encountered in implantation of the device including therapy-delivery leads and even in application of the therapy for the benefit of the large numbers of patients who were identified as real candidates for such treatment.
In the case of cardiac pacing, the implant procedure itself was the major obstacle. Further, the technology was relatively primitive at its beginning, which precluded a more wide-spread use including prophylactic use for patients whose needs were not perceived as crucial but who, on the basis of prognosis, could derive benefit from the availability of demand-type pacing therapy.
The same situation exists for defibrillation therapy today. Difficult implant procedures such as open chest surgery necessitated for epicardial patch electrodes, coupled with the large size and relatively high cost of the devices, and the complex medical procedures associated with the implantation and difficulty of follow-up procedures for the patients, place serious limitations on the number of patients who can actually be addressed by the therapy in contrast to the significantly larger number of current patients who could benefit from implanted defibrillators for cardioversion and defibrillation of the heart.
While problems encountered with implantable defibrillators are somewhat ameliorated by the now growing clinical use of transvenous defibrillation leads, the current need for high energies to defibrillate the heart dictate capacitor and battery sizes that continue to mandate a rather uncomfortable device with a size and a weight unpleasant to patients. It follows that the major contribution to a more widespread use will be a substantial reduction in device size and weight and by facilitation of the implant procedure.
To reduce the size of the defibrillator, it is necessary to considerably reduce the energy required to defibrillate the heart (the so-called defibrillation threshold DFT). Current devices require more than 32 joules of energy, and even with further optimization that might be achieved with current lead and patch technology, in the majority of cases a DFT of 18 or more joules would still be needed. Defibrillation thresholds of that magnitude mean that despite improvements in implant technology, such as in capacitors and other larger-sized components and higher packing density in the devices, the device size and weight may only be reduced to a minor extent. The major breakthrough will only result from a considerable reduction in DFT.
Studies conducted by a research group with which the applicant herein is associated have revealed that the use of new low-polarization fiber materials for the defibrillation electrodes, such as are described in the '089 patent and the '173 application, considerably increase the effective electrical surface over the actual geometrical surface of the electrode. The result can be a significant reduction in DFT.
Of course, microminiaturization of electronic components has brought about considerable size reduction of implantable medical devices adapted to monitor and/or deliver electrical indicia and/or stimuli (broadly, electrical signals) within the body to detect or stimulate selected biological processes. Implantable defibrillators are broadly capable of monitoring and controlling cardiac activity as necessary to detect and alleviate arrhythmias or dysrhythmias such as fibrillation (both atrial defibrillation (AF) and ventricular fibrillation (VF)), pathologic tachycardia, and, in some instances, bradycardia. These devices respond automatically to sensing the dysrhythmia of interest to restore normal, regular cardiac rhythm by delivering electrical stimuli in the form of pulses, shocks or more complex waveforms to the atrial or ventricular cardiac tissue, as appropriate.
The invention described in the '089 patent is an improvement in defibrillating or cardioverting electrodes for automatic implantable defibrillators. The prior art had taught implanting patch electrodes to produce an electric field extending from one side of the heart to the other, e.g., through the ventricles, or a combination of an epicardial patch or patches and an endocardial counter-electrode implanted transvenously. Implantation of external patch electrodes had required a thoracotomy, which is highly invasive and requires a long recovery period.
An article titled "Clinical experience, complications and survival in 70 patients with the automatic implantable cardioverter/defibrillator", Circulations, vol. 71, no. 2, 1985, pp. 289-296 (D. Echt et al.) surveys complications associated with implantation of epicardial patch electrodes.
Another piece of prior art of interest is U.S. Pat. No. 4,774,952 which describes a multiple electrode structure that concentrates current in selected areas of the heart during defibrillation.
Problems persisted in attempts to interface electrodes with body tissue to attain a combination of low resistance conductivity, large surface area, low polarization and low intrinsic stiffness, while providing long-term communication of electrical signals and relatively high current levels. The implantation and manipulation of prior art electrodes remained complex procedures, particularly those employed for defibrillation, and were not very successful. A principal reason was the inefficient energy transfer between the electrode and excitable tissue (i.e., tissue having cell membrane field strength which can be stimulated electrically to produce cell depolarization).
Energy transmission and transfer between pacemaker electrodes and the heart has received considerable attention (see, e.g., survey article of Ripart and Muciga titled "Electrode heart interface: definition of the ideal electrode", in PACE, vol. 6, March 1983, pp. 410-421). Encouraging results may have been obtained using low polarization materials such as platinum, iridium and pyrolyzed carbon in the electrode tip having average surface area of 10 mm.sup.2 (square millimeters), for stimulation with pacing pulses ranging from 2.5 to 5.0 volts, and for sensing cardiac activity. But cardiac pacing electrode requirements are manifestly different from defibrillation electrode requirements, which involve application of shocks with amplitudes of up to many hundreds of volts and over electrode surface areas of perhaps 10,000 mm.sup.2.
German Democratic Republic patent 26 32 39 of Oct. 30, 1987 disclosed a pacing lead composed of a bundle of anisotropic carbon fibers. The electrode structure may be effective for pulse transmission along its longitudinal axis, but its relatively tiny point contact surface areas at the tissue interface and the tendency of the electrode interface to erode over time make it incapable of providing the large surface area required for defibrillation. Transfer of adequate energy from defibrillating pulses of from 500 to 2,000 volts typically required patch electrode surface area of from 50 to 100 cm.sup.2 (square centimeters) to avoid possible local burning of the tissue, and transvenous electrode surface area of from 4 to 20 cm.sup.2 for uniform energy flow through the heart.
The invention disclosed in the '089 patent provides a defibrillating electrode of low energy consumption, low polarization, flexibility to avoid mechanical restriction of the heart during beating, and ease of implantation. Implantation is performed without thoracotomy, long patient recovery time, pericardial hemorrhaging, potential infection, or complications attributable to adhesions from prior surgery where the patient is undergoing electrode replacement. Simplified implantation also reduces hospital costs by eliminating a need for specialized open heart surgical facilities and attending personnel.
According to the '089 patent, the electrode is composed of flexible, nonmetallic, electrically conductive, uninsulated fiber strands possessing isotropic conductance characteristics to form an improved electrical interface with body fluid (e.g., blood) and/or excitable tissue when the lead assembly is implanted into the body. Improved energy transfer and electrical communication across the electron-ion interface between the electrode and the blood/tissue, formed along the entire length of exposed surface of each fiber rather than at restricted point contact areas, are attained. The isotropic property assures electron conduction substantially equally in all directions, thereby expanding the electron-ion boundary.
By utilizing a multiplicity of such fibers in the electrode, its effective electrical surface area is many times larger than its actual geometrical surface area. The fibers allow the electrode to be configured in a size and shape suitable for the interface at the selected implant location.
Other prior art relevant to the '089 patent invention includes the following. U.S. Pat. No. 4,574,814 (Buffet) describes synchronous pacing in chambers of different size, using resiliently deformable carbon fibers which assume a "feather duster" shape with fiber ends providing an envelope of contact of myocardial tissue to increase the area of anchoring, and point contact of non-isotropic fibers with the tissue for excitation and anchoring. An article titled "Carbon Fibers as an Electrode Material" (Starrenburg et al.) in IEEE Transactions on Biomedical Engineering, vol. BME-29, no. 5 (May 1982), at pp. 352 et seq., describes a flexible carbon fiber bundle electrode having a short segment of straight bare fibers between two insulated regions for muscle stimulation, but notes that the electrode suffered breakdown during electrical pulse testing and as a result of mechanical stresses when implanted. An article titled "New Plastics That Carry Electricity" in the Jun. 18, 1979 issue of Newsweek magazine describes the possible use of polyacetylene plastic doped with chemicals to enable it to carry electric current, as a pacemaker lead wire.
Great Britain patent No. 1,219,017 (Thomson Medical-Telco) describes an electrical conductor of braided ,non-isotropic, carbonized fibers as a lead for cardiac pacing, which is insulated along its entire length except at an end for point contact excitation of tissue. U.S. Pat. No. 4,506.673 (Bonnell) describes electrical tissue growth stimulators in which a non-isotropic mesh of biodegradable, electrically conductive, carbon particle-impregnated cotton fibers provides cathodic and anodic stimulation. U.S. Pat. No. 4,198,991 (Harris) describes a cardiac pacing lead with carbon filaments covered by an insulating sheath except at the lead tip the form a brush-like electrode structure for point contact of tissue.
The '173 application discloses a patch, tube or other mesh of individually woven isotropic carbon fibers or metallic fibers coated with isotropic carbon employed as an electrode for cardioversion or defibrillation. Such electrodes impregnated with anticoagulant substance or pharmacologic agent, such as heparin or hirudin, prevent blood clotting or platelet formation along the fibers which would decrease the efficiency of the electrode in energy transfer vis-a-vis the cardiac tissue to be stimulated.
Such isotropic carbon or carbon-coated metal fiber defibrillation electrodes may be implanted with minimal invasive surgery, as by a puncture opening in the abdomen or chest wall. Electrode insertion is made with a puncture needle and associated introducing catheter, into the pericardial sac for positioning adjacent the epicardium. A braided conductive fiber (carbon or carbon-coated) tube, preformed into a flat coil configuration, is readily inserted through the puncture using a stiffening wire.
To further improve the implantation and operation of these defibrillator devices, it is desirable to make use of the defibrillator can (i.e., the metal housing of the implantable waveform generator) as one of the electrical poles of the overall therapy delivery system, and predominantly as the anodal pole. In this way, a right ventricular defibrillation lead which is implanted completely transvenously and without major surgery, similar to a implanting a pacing lead, acts as one of the electrodes, and the defibrillator can acts as the second electrode. Although this will facilitate implant technology and surgical requirements considerably, it will not measurably reduce the size and cost of the devices.
It is a principal object of the present invention to provide means for adequately reducing the defibrillation threshold without the need for major surgery, and in that respect, to provide an implantable defibrillator having a weight of approximately 50 grams which would therefore be much better matched to patient need and comfort.
It is a further object to reduce the cost of the device and its implantation. This is an important consideration in view of the increasing concern over health care cost.